Mixed-mode, high-cycle fatigue-crack growth thresholds in Ti–6Al–4V

The role of crack-tip shielding in influencing mixed-mode (mode I+II) fatigue-crack growth thresholds for large, through-thickness cracks in a Ti–6Al–4V turbine blade alloy is examined under high-cycle fatigue loading conditions, i.e., at a loading frequency of 1000 Hz in ambient temperature air for load ratios (K min/K max) of R=0.1–0.8. Techniques are developed to quantify crack-tip shielding with respect to both the mode I and mode II applied loading, enabling an estimation of the shielding-corrected, crack-driving forces actually experienced at the crack tip (ΔK I,TH,eff and ΔK II,TH,eff or ΔG TH,eff). In Part I, it was shown that when the crack-driving force is characterized in terms of the range in strain-energy release rate, ΔG, which incorporates contributions from both the applied tensile and shear loading, the mixed-mode (I+II) fatigue-crack growth resistance increases monotonically with the ratio ΔK II/ΔK I. When the fatigue-crack growth thresholds are expressed in terms of the near-tip (shielding-corrected) crack-driving force, this increase in crack-growth resistance with increasing mode mixity is markedly reduced. Moreover, for all mode mixities investigated, the near-tip mixed-mode fatigue threshold is lower than the applied (global) value, with the effect being particularly pronounced under shear-dominant loading conditions. These observations illustrate the prominent role of crack-tip shielding for the mixed-mode loading of fatigue cracks with crack-wake dimensions large compared with microstructural size scales; specifically, they indicate that the elevation of the ΔG TH fatigue-crack growth threshold with increasing applied mode mixity is largely due to a shear-induced enhancement of crack-tip shielding.